my %order;
while ( my $rec = $data->fetchrow_hashref ) {
    push @{ $result{ $rec->{"ID"} } }, $rec->{"item"};
    push @order, $rec->{ID};                           # read order
}

##</code><code>##

my $last = $order[0];                                  # first key

for my $cur ( @order[1..$#order] ) {                   # following keys
   compare($last,$cur);
   $last = $cur;                                       # keep current as last
}

##</code><code>##

sub compare {
  my ($previous,$present) = @_;
  ...
}